Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbomachines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles in the combustors where the compressed working fluid mixes with fuel before igniting in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Combustion instabilities may occur during operation when one or more acoustic modes of the gas turbine are excited by the combustion process. The excited acoustic modes may result in periodic oscillations of system parameters (e.g., velocity, temperature, pressure) and processes (e.g., reaction rate, heat transfer rate). For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel-air ratio fluctuation in the flame. When the resulting fuel/air ratio fluctuation and the acoustic pressure pulsations have a certain phase behavior (e.g., approximately in-phase), a self-excited feedback loop may result. This mechanism, and the resulting magnitude of the combustion dynamics, depends at least in part on the delay between the time that the fuel is injected through the fuel nozzles and the time when the fuel reaches the combustion chamber and ignites, defined as convective time (Tau). When the convective time increases, the frequency of the combustion instabilities decreases, and when the convective time decreases, the frequency of the combustion instabilities increases.
The resulting combustion dynamics may reduce the useful life of one or more combustor and/or downstream components. Therefore, a fuel nozzle that varies the convective time would be useful to enhancing the thermodynamic efficiency of the combustors, protecting against accelerated wear, promoting flame stability, and/or reducing undesirable emissions over a wide range of operating levels.